Immune Reconstitution Inflammatory Syndrome (IRIS) is the rapid and paradoxical worsening of pathology seen in HIV-1 infected individuals after initiation of anti-retroviral therapy (ART). With the increasing use of ART for mass treatment of AIDS in developing countries, this complication has emerged as a major problem in the management of HIV infection. IRIS is more common in patients with severe CD4 T cell lymphopenia and patients with mycobacterial and other opportunistic infection are at particularly high risk of developing this severe pathologic response. The factors that lead to the induction of IRIS and the mechanisms of its immunopathogenesis are poorly understood, so currently patient treatment during IRIS episodes is limited to supportive care and broad steroid based immunosuppression. We have been engaged in both experimental and clinical studies investigating the mechanisms underlying the immunopathogenesis of IRIS in order to identify patients at risk and to design alternative interventions. As introduced in a previous report we have developed a murine model which recapitulates the key features of the IRIS disease occuring in ART treated HIV patients with mycobacterial co-infections. In this model originated by Dan Barber in the lab and published last year (Barber et al., Blood, 2010) we utilize T cell deficient TCR alpha -/- mice infected with M. avium, an important opportunistic pathogen associated with human IRIS. Following adoptive transfer with CD4+ T cells these animals develop a severe wasting disease with impaired lung function and rapidly succumb. In analyzing this form of immune reconstitution disease (IRD), we found that it requires Ag recognition and IFN-gamma production by the donor CD4 T cells, and correlates with marked alterations in blood and tissue CD11b+ myeloid cells. In new work completed this year, Dr. Barber has shown that the CD4+T cells that transfer disease do not require Tbet, STAT1, and IFNgammaR, three factors critical for Th1 cell development and therefore may not represent classical Th1 effectors despite their IFN-gamma dependent function. To further analyze the role of myeloid cells in murine IRIS, he has used G-CSF blockade to prevent the neutrophil response in blood and lung tissue. Interestingly this had no effect on the wasting disease and mortality occurring in the model. In contrast, neutralization of IL-6, a proinflammatory cytokine previously shown to be elevated in IRIS episodes, caused a major amelioration of disease and mortality following T cell transfer. As summarized in previous reports, we have also been engaged in a collaborative clinical study with Drs.Irini Sereti and Mario Roederer aimed at identifying immunologic correlates of IRIS. In this project conducted at the NIH clinical center we characterized T cell phenotypic markers and serum cytokine levels in 45 HIV+ patients with a range of different AIDS defining illnesses, before and at regular time-points after initiation of ART. In the first part of the study focusing on baseline responses in the absence of in vitro restimulation which was published last year (Antonelli et al., 2010) we reported that patients developing IRIS episodes displayed higher frequencies of effector memory, PD-1+, HLA-DR+ and Ki67+ CD4+ T cells than patients without IRIS. Moreover, PD-1+ CD4+ T cells in IRIS patients expressed increased levels of LAG-3, CTLA-4 and ICOS and had a Th1/Th17 skewed cytokine profile upon polyclonal stimulation. These findings suggested that IRIS is a predominantly CD4-dependent phenomenon with reconstituting effector cells displaying hyperactivation possibly from increased antigenic exposure. The second part of this study completed this year by our collaborator Yolanda Mahnke (manuscript submitted) analyzed responses of cells from the same patients following restimulation with antigens from the major opportunistic infection associated with each HIV case. Exuberant inflammatory T-cell responses directed specifically to these antigens emerged around the time of clinical IRIS episode. These boosted responses were largely restricted to the CD4+ T-cell compartment and in most cases were also detectable pre-ART. The findings thus support our earlier hypothesis that IRIS is associated with increased activation of CD4+T cell responses driven by specific Ag exposure rather than due to a global non-specific alteration in T cell responsiveness occuring in the affected patients. Type I IFNs have been shown to exacerbate tuberculosis in mice and to be associated with disease progression in infected humans. Nevertheless, the mechanisms by which type I IFNs regulate the host response to M. tuberculosis infection are poorly understood and particularly so in humans. As introduced in last years report and recently published by us (Novikov et al 2011) M. tuberculosis induces an IFN-related gene expression signature in infected primary human macrophages, which is dependent on host type I IFN signaling as well as the mycobacterial virulence factor, region of difference-1. We further showed that type I IFNs selectively limit the production of IL-1&#946;, a critical mediator of immunity to M. tuberculosis, by human macrophages. This regulation occurs at the level of IL1B mRNA expression, rather than caspase-1 activation or autocrine IL-1 amplification and appears to be preferentially used by virulent mycobacteria since avirulent M. bovis bacillus Calmette-Gurin (BCG) fails to trigger significant expression of type I IFNs or release of mature IL-1&#946;protein. These findings demonstrate that virulent and avirulent mycobacteria employ distinct pathways for regulating IL-1&#946;production in human macrophages and reveal that in the case of M. tuberculosis infection the induction of type I IFNs is a major mechanism mediating this control.